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A Conceptual Perspective on Processes That Deposit Mud and Their Impact on Shale Fabrics and Future Pore Systems

Abstract

The rock record is dominated by shales and mudstones, an increasingly important rock class for the production of fossil fuels. Experimental studies of the processes by which these sediments are transported and deposited are changing our understanding of mudstone depositional systems in significant ways. For example: Whereas quiet settling in low energy environments used to be the “conventional” perspective, we now know now that flocculated muds can accumulate in form of bedload ripples at flow velocities that would suffice for bedload transport of sand. Both low energy settling and bottom current transport produce laminated sediments, but the character of these laminae differs in detail, reflecting the difference in floccule structure between (A) settled deposits (very loose floccules with easily collapsed fabric) and (B) current transported floccules (rolled floccules with significantly lower water content). Compaction of these deposits will in case (A) result in well aligned clays, minimal development if intergranular porosity, well developed seal characteristics, and a poor reservoir. In case (B) the higher initial packing of clays will tend to lead to poorly aligned clays, development of framework porosity beween clay flakes, and a more favorable reservoir facies. Insights like these suggest that with further experimental work we may eventually arrive at a comprehensive understanding of mudstone sedimentology, initial mudstone fabrics, and their downstream pore systems. A higher level of understanding fine grained sediments is the “next step” for making meaningful progress in the study of mudstone hosted energy resources. Flume studies provide a physical basis for interpreting sedimentary structures in shales, and that in turn leads to greatly improved EOD assessment in shale successions. As shale fabrics become meaningful, it also becomes apparent that depositional mode has a direct impact on the developing sedimentary fabric, shale microstructure, and initial pore development. High resolution petrography shows that primary pore structures may partially survive burial and can be critical to the permeability and producability of unconventional reservoirs. In that context, flume studies not only open up new avenues of inquiry for depositional processes and microfabrics, but also provide alternative scenarios of carbon burial and the origin of unconventional hydrocarbon resources.